This invention relates generally to white noise generators and more particularly to a white noise generator which takes advantage of the thermal noise voltage of a resistor connected in a feedback arrangement with a special inverting amplifier to provide a wideband, stable, random, white noise source with a high level and well-defined output power spectral density.
White noise has been defined as random noise having constant energy per unit bandwidth that is independent of the central frequency of the band. The oldest and probably best known source of high-level white noise is the emission-limited vacuum diode. A low noise amplifier is most often used to boost the diodes noise output to a higher level but the frequency response is usually degraded in the process. While this might be overcome, the major problem with the emission-limited diode is that it is an active device whose characteristics are likely to change with time. The cathode, for example, is subject to aging effects. Although the theoretical equations for predicting noise output are well-established, noise generators based on the emission-limited diode are not in common use today.
A gas tube, as a noise source, has one important advantage over the emission-limited diode in that it has a very large noise output without the requirements of substantial amplification. It is also operable up to very high frequencies. On the other hand it is subject to aging caused by the fairly high current carried by the tube; also the equations do not exist that permit the tube's performance to be predicted to be better than about .+-.10%. As a result, gas tubes are generally used in noise generators having accuracies in the range of from 1 to 5%.
A zener diode is basically the solid-state realization of the gas tube. Like the gas tube, its output noise level is adequately high but it carries a high current, is subject to change with age, and the noise equations, likewise, are not well-established. Therefore, its performance is also not very predictable.
A fourth class of devices are the random noise generators that utilize digital output signals. These devices are not fully digital but are based on gas tubes, etc., and the circuits have much the same limitations as the noise sources they incorporate.
There has long been a need in the instrumentation field, especially in the calibration of signal processing systems including nuclear pulse instruments, operational and other amplifiers, reactor noise spectrum analyzers, Johnson Noise Power Thermometers, etc., for a highly predictable white noise source that overcomes the limitations of existing generators. A simple resistor having negligible DC current flow is probably the most predictable noise source known. Such a resistor exhibits pure thermal noise that is well characterized by thermodynamic principals, and is predictable to very high accuracy up to frequencies of 20 gigahertz or more. The open-circuit noise voltage of a resistor has been well defined mathematically. The mean-squared value of the open-circuit thermal noise voltage of a resistor of known ohmic value and known temperature can be calculated if the resistor is conducting no current. The mean-squared thermal noise voltage can be computed from the Nyquist formula, EQU E.sub.n.sup.2 =4kTR.DELTA.f
where E.sub.n.sup.2 is the root mean-squared (RMS) thermal noise voltage, k is the Boltzmann constant, T is the absolute temperature in Kelvins, R is the resistance in ohms, and .DELTA.f is the noise bandwidth in hertz. This equation is applicable up to very high frequencies (several GHz).
Since the use of a resistor as a high-level noise source requires a high value for R, an attempt to obtain the open-circuit noise voltage can result in severe loading with the usual consequence of strong attenuation (perhaps by stray and amplifier input capacitances) of the high-frequency noise voltage of the resistor. This in turn results in noise output levels that are generally too low to be of any practical value in measurements, such as gain, gain-bandwidth constants, etc., that are expected of the noise generator.
If some DC current is flowing through the resistor, there is a possibility of an additional noise source, known as shot noise, that is due primarily to the way the resistor is made. Metal film resistors have practically no shot noise even with a small DC current flow. Thus, in order to use a resistor as a noise source it would be of primary importance to arrange a high quality metal film resistor in a circuit having practically no DC current flow through the resistor.